Propylene polymers are excellent in strength, heat resistance, molding workability, chemical resistance, and also in economical efficiency, and have wide applications in household appliance parts, automotive parts and building components.
The propylene polymers mentioned above are required to have a further improved impact strength at low temperature in the fields of household appliance parts and automotive parts. For example, the impact resistance is imparted to the propylene polymers by incorporation of an ethylene propylene copolymer rubber into said propylene polymers.
Though the impact strength at low temperature can be imparted sufficiently to the propylene polymers by incorporating thereinto large amounts of the ethylene propylene copolymer rubber, the thus incorporated propylene polymers tend to decrease in stiffness properties and surface hardness. In order to prevent such a decrease in balance of physical properties as mentioned above, various attempts have been made, such as varying the composition of the ethylene, propylene copolymer rubber to be incorporated into the propylene polymers, or adding fillers thereto. For example, Japanese Patent Laid-Open Publn. No. 256856/1987 discloses propylene polymer compositions comprising a block copolymer consisting of propylene and ethylene, an ethylene/.alpha.-olefin random copolymer rubber, and talc or calcium carbonate, which are alleged to be excellent in impact resistance (impact strength at low temperature), stiffness properties and coating properties.
Further, Japanese Patent Laid-Open Publn. No. 42852/1990 discloses polypropylene resin compositions comprising an ethylene/propylene block copolymer, an ethylene/propylene copolymer rubber, and an ethylene/propylene/diene terpolymer rubber and talc, which are said to be excellent in stiffness properties and impact resistance.
Furthermore, Japanese Patent Laid-Open Publn. No. 33814/1991 discloses polypropylene resin compositions comprising an ethylene/propylene block copolymer, an ethylene/propylene copolymer rubber, an ethylene/propylene/diene terpolymer rubber and an inorganic filler such as talc, which are alleged to be excellent in stiffness properties, impact resistance and molding fluidity.
By the way, though molded products formed from such propylene polymer composition as mentioned above, for example, automotive bumpers are excellent in impact resistance, they are low in surface hardness. Accordingly, there is sometimes such a case wherein the bumper surface is marred at the time of molding or transporting thereof, and the bumper results in a rejected product. Accordingly, propylene polymer compositions capable of providing molded products having a further high surface hardness, that is, the molded products excellent in scratch resistance, are required.
The present inventors conducted extensive researches into propylene polymer compositions capable of forming molded products excellent in impact resistance as well as in scratch resistance without the sacrifice of excellent characteristics of propylene polymer, and eventually have found that propylene polymer compositions comprising a specific propylene block copolymer, and a specific ethylene-propylene copolymer rubber, a specific ethylene-propylene-diene copolymer rubber, a specific ethylene-butene-1 copolymer rubber and talc in specific proportions are capable of forming molded products having excellent impact strength at low temperature as well as in scratch resistance without the sacrifice of excellent characteristics inherent in propylene polymer, leading to accomplishment of the present invention.